RANDOM(4)                 (2017-09-15)                  RANDOM(4)

     NAME
          random, urandom - kernel random number source devices

     SYNOPSIS
          #include <linux/random.h>

          int ioctl(fd, RNDrequest, param

     DESCRIPTION
          The character special files /dev/random and /dev/urandom
          (present since Linux 1.3.30) provide an interface to the
          kernel's random number generator.  The file /dev/random has
          major device number 1 and minor device number 8.  The file
          /dev/urandom has major device number 1 and minor device num-
          ber 9.

          The random number generator gathers environmental noise from
          device drivers and other sources into an entropy pool.  The
          generator also keeps an estimate of the number of bits of
          noise in the entropy pool.  From this entropy pool, random
          numbers are created.

          Linux 3.17 and later provides the simpler and safer
          getrandom(2) interface which requires no special files; see
          the getrandom(2) manual page for details.

          When read, the /dev/urandom device returns random bytes
          using a pseudorandom number generator seeded from the
          entropy pool.  Reads from this device do not block (i.e.,
          the CPU is not yielded), but can incur an appreciable delay
          when requesting large amounts of data.

          When read during early boot time, /dev/urandom may return
          data prior to the entropy pool being initialized.  If this
          is of concern in your application, use getrandom(2) or
          /dev/random instead.

          The /dev/random device is a legacy interface which dates
          back to a time where the cryptographic primitives used in
          the implementation of /dev/urandom were not widely trusted.
          It will return random bytes only within the estimated number
          of bits of fresh noise in the entropy pool, blocking if nec-
          essary.  /dev/random is suitable for applications that need
          high quality randomness, and can afford indeterminate
          delays.

          When the entropy pool is empty, reads from /dev/random will
          block until additional environmental noise is gathered.  If
          open(2) is called for /dev/random with the O_NONBLOCK flag,
          a subsequent read(2) will not block if the requested number

     Page 1                        Linux             (printed 5/24/22)

     RANDOM(4)                 (2017-09-15)                  RANDOM(4)

          of bytes is not available.  Instead, the available bytes are
          returned.  If no byte is available, read(2) will return -1
          and errno will be set to EAGAIN.

          The O_NONBLOCK flag has no effect when opening /dev/urandom.
          When calling read(2) for the device /dev/urandom, reads of
          up to 256 bytes will return as many bytes as are requested
          and will not be interrupted by a signal handler.  Reads with
          a buffer over this limit may return less than the requested
          number of bytes or fail with the error EINTR, if interrupted
          by a signal handler.

          Since Linux 3.16, a read(2) from /dev/urandom will return at
          most 32 MB.  A read(2) from /dev/random will return at most
          512 bytes (340 bytes on Linux kernels before version
          2.6.12).

          Writing to /dev/random or /dev/urandom will update the
          entropy pool with the data written, but this will not result
          in a higher entropy count.  This means that it will impact
          the contents read from both files, but it will not make
          reads from /dev/random faster.

        Usage
          The /dev/random interface is considered a legacy interface,
          and /dev/urandom is preferred and sufficient in all use
          cases, with the exception of applications which require ran-
          domness during early boot time; for these applications,
          getrandom(2) must be used instead, because it will block
          until the entropy pool is initialized.

          If a seed file is saved across reboots as recommended below,
          the output is cryptographically secure against attackers
          without local root access as soon as it is reloaded in the
          boot sequence, and perfectly adequate for network encryption
          session keys.  (All major Linux distributions have saved the
          seed file across reboots since 2000 at least.)  Since reads
          from /dev/random may block, users will usually want to open
          it in nonblocking mode (or perform a read with timeout), and
          provide some sort of user notification if the desired
          entropy is not immediately available.

        Configuration
          If your system does not have /dev/random and /dev/urandom
          created already, they can be created with the following com-
          mands:

              mknod -m 666 /dev/random c 1 8
              mknod -m 666 /dev/urandom c 1 9
              chown root:root /dev/random /dev/urandom

          When a Linux system starts up without much operator

     Page 2                        Linux             (printed 5/24/22)

     RANDOM(4)                 (2017-09-15)                  RANDOM(4)

          interaction, the entropy pool may be in a fairly predictable
          state.  This reduces the actual amount of noise in the
          entropy pool below the estimate.  In order to counteract
          this effect, it helps to carry entropy pool information
          across shut-downs and start-ups.  To do this, add the lines
          to an appropriate script which is run during the Linux sys-
          tem start-up sequence:

              echo "Initializing random number generator..."
              random_seed=/var/run/random-seed
              # Carry a random seed from start-up to start-up
              # Load and then save the whole entropy pool
              if [ -f $random_seed ]; then
                  cat $random_seed >/dev/urandom
              else
                  touch $random_seed
              fi
              chmod 600 $random_seed
              poolfile=/proc/sys/kernel/random/poolsize
              [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
              bytes=$(expr $bits / 8)
              dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

          Also, add the following lines in an appropriate script which
          is run during the Linux system shutdown:

              # Carry a random seed from shut-down to start-up
              # Save the whole entropy pool
              echo "Saving random seed..."
              random_seed=/var/run/random-seed
              touch $random_seed
              chmod 600 $random_seed
              poolfile=/proc/sys/kernel/random/poolsize
              [ -r $poolfile ] && bits=$(cat $poolfile) || bits=4096
              bytes=$(expr $bits / 8)
              dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

          In the above examples, we assume Linux 2.6.0 or later, where
          /proc/sys/kernel/random/poolsize returns the size of the
          entropy pool in bits (see below).

        /proc interfaces
          The files in the directory /proc/sys/kernel/random (present
          since 2.3.16) provide additional information about the
          /dev/random device:

          entropy_avail
               This read-only file gives the available entropy, in
               bits.  This will be a number in the range 0 to 4096.

          poolsize
               This file gives the size of the entropy pool.  The

     Page 3                        Linux             (printed 5/24/22)

     RANDOM(4)                 (2017-09-15)                  RANDOM(4)

               semantics of this file vary across kernel versions:

               Linux 2.4:
                    This file gives the size of the entropy pool in
                    bytes. Normally, this file will have the value
                    512, but it is writable, and can be changed to any
                    value for which an algorithm is available.  The
                    choices are 32, 64, 128, 256, 512, 1024, or 2048.

               Linux 2.6 and later:
                    This file is read-only, and gives the size of the
                    entropy pool in bits. It contains the value 4096.

          read_wakeup_threshold
               This file contains the number of bits of entropy
               required for waking up processes that sleep waiting for
               entropy from /dev/random. The default is 64.

          write_wakeup_threshold
               This file contains the number of bits of entropy below
               which we wake up processes that do a select(2) or
               poll(2) for write access to /dev/random. These values
               can be changed by writing to the files.

            boot_id
               These read-only files contain random strings like
               6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9.  The former is
               generated afresh for each read, the latter was gener-
               ated once.

        ioctl(2) interface
          The following ioctl(2) requests are defined on file descrip-
          tors connected to either /dev/random or /dev/urandom.  All
          requests performed will interact with the input entropy pool
          impacting both /dev/random and /dev/urandom.  The
          CAP_SYS_ADMIN capability is required for all requests except
          RNDGETENTCNT.

          RNDGETENTCNT
               Retrieve the entropy count of the input pool, the con-
               tents will be the same as the entropy_avail file under
               proc.  The result will be stored in the int pointed to
               by the argument.

          RNDADDTOENTCNT
               Increment or decrement the entropy count of the input
               pool by the value pointed to by the argument.

          RNDGETPOOL
               Removed in Linux 2.6.9.

          RNDADDENTROPY

     Page 4                        Linux             (printed 5/24/22)

     RANDOM(4)                 (2017-09-15)                  RANDOM(4)

               Add some additional entropy to the input pool, incre-
               menting the entropy count.  This differs from writing
               to /dev/random or /dev/urandom, which only adds some
               data but does not increment the entropy count.  The
               following structure is used:

                   struct rand_pool_info {
                       int    entropy_count;
                       int    buf_size;
                       __u32  buf[0];
                   };

               Here entropy_count is the value added to (or subtracted
               from) the entropy count, and buf is the buffer of size
               buf_size which gets added to the entropy pool.

          RNDZAPENTCNT, RNDCLEARPOOL
               Zero the entropy count of all pools and add some system
               data (such as wall clock) to the pools.

     FILES
          /dev/random
          /dev/urandom

     NOTES
          For an overview and comparison of the various interfaces
          that can be used to obtain randomness, see random(7).

     BUGS
          During early boot time, reads from /dev/urandom may return
          data prior to the entropy pool being initialized.

     SEE ALSO
          mknod(1), getrandom(2), random(7)

          RFC 1750, "Randomness Recommendations for Security"

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 5                        Linux             (printed 5/24/22)